The present invention relates to the art of contactless magnetic position sensors with magnetosensitive probe and permanent magnet.
Position sensors using Hall-effect probes to detect the magnetic flux generated by the relative displacement of a permanent magnet referenced to a Hall-effect probe are known in the prior art.
In particular, French Patent No. 2624966 describes a coder for a printwheel comprising a linear Hall-effect detector and a permanent magnet mounted in a nonferrous metal shaft provided with a bush forming a flux-conducting spiral molded in the wheel and encircling the shaft. The angular position of the printwheel is determined in absolute value by the signal amplitude relative to the transition point of the spiral. The sensor designed in this way does not provide a truly linear signal. According to one proposal in the prior art, this lack of linearity can be remedied by digitizing the signal delivered by the Hall-effect probe and processing the signal by information-processing means.
According to another proposal in the prior art, the lack of linearity of magnetic position sensors can be remedied by complex geometries. These solutions are technically difficult to implement for sensors produced industrially in large quantities. Consequently, the manufacturing cost is excessive in numerous applications.
Also proposed in the prior art are sensors having a movable part formed by a ferromagnetic yoke on which a permanent magnet of angular width in excess of 180xc2x0 is directly fixed, the complementary part being formed by a ferromagnetic material, or by a permanent magnet of inverse polarity or by the air. Such a solution is proposed in French Patent Application 2739444. Since the angular width of the magnet is greater than 180xc2x0, it entails a large magnet volume, which is overdimensioned for the measurement of short strokes.
Solutions with two permanent magnets of alternate polarity and/or a plurality of Hall-effect probes have also been proposed. As an example, such solutions are proposed in French Patent Applications 2670286 and 2715726. These structures use multipole magnets or assemblies of a plurality of magnets having alternate directions, which precludes magnetization after assembly and makes it necessary to manipulate high-energy magnets during assembly of the sensors. Such manipulation requires great care, because high-energy magnets can introduce particles such as metal cuttings into the sensors while they are being placed in position. It is known that this drawback can be prevented by encapsulating the magnets, but such a solution has the drawback of further increasing the cost price of the sensors.
French Patent 2691534 describes a linear position sensor provided with a single movable two-pole permanent magnet inside a thin primary air gap bounded by two stator parts. This sensor necessitates a guide system for displacement of the permanent magnet. The necessary magnet volume and the linearity are not optimum. Consequently, in some exacting applications, this sensor has a cost and characteristics which are deemed to be unacceptable.
The prior art sensors use permanent magnets which generally contain rare earths of the neodymium-iron-boron or samarium-cobalt type. These high-performance magnets are still relatively expensive. Consequently the prior art structures have a relatively high cost.
The linearity of the signal delivered by the sensor as a function of position is a fundamental characteristic that determines the quality of a position sensor. In the ideal case, this function is a straight line.
The sensors cited hereinabove all use two transformations:
The first transformation is performed by the ferromagnetic circuit. This makes it possible to deliver to a magnetosensitive measuring component, such as a Hall-effect probe, a magnetic induction that varies as a function of position. The first transformation is then determined by the induction (position) characteristic.
The second transformation is performed by the measuring element inserted in the magnetic circuit. It delivers an output signal, generally in the form of a voltage, which varies with the amplitude of the induction in which it is placed. The second transformation is then determined by the voltage (induction) characteristic.
To obtain a sensor with very great linearity, it is desirable to perform two linear transformations with the greatest possible linearity.
If one of the transformations does not have more or less good linearity, it is necessary to compensate for its linearity error with the other transformation or by means of electronic and/or information-processing devices, which is costly and inelegant.
The linearity of the second transformation depends on the quality of the measuring element used. At present, commercial Hall-effect probes can be found that have a linearity error of smaller than xc2x10.2% over the characteristic (output signal/induction).
Very great linearity of the first transformation can be achieved by judicious design of the magnetic circuit, and this is the object of the present invention.
Another important characteristic of a position sensor is the amplitude of the signal-to-noise ratio. To construct a high-quality magnetic position sensor, it is necessary in practice to achieve a magnetic circuit that delivers a sufficiently large induction variation xcex94B as a function of position that a high signal-to-noise ratio is obtained. This partly determines the volume and quality of the magnet to be used.
It must be noted that the use of an excessively large induction variation is not recommended. This in fact entails extra costs for the magnet. Moreover, the linearity of a Hall-effect probe (second transformation) deteriorates for high values of induction.
The object of the present invention is to provide an improved contactless magnetic position sensor of lower cost and great reliability, exhibiting great linearity together with an optimal magnet volume.
To this end, the invention relates in its more general embodiment to a magnetic position sensor provided with a stator and at least one part that is movable in at least one direction OX, with a useful stroke Xc,
[Xc represents, in the case of a rotary sensor, the width of the angular arc traveled by the movable part over the mean radius of a magnetized part and, in the case of a linear sensor, the stroke of the movable part in a direction contained in the central plane of the primary air gap],
the stator being composed of at least two pieces of soft magnetic material defining at least one secondary air gap in which there is housed at least one magnetosensitive probe for measuring the variation of induction in this secondary air gap, each stator part having a length Xs equal to at least Xc in the direction OX,
[Xs being measured over the mean radius of a magnet in the case of a rotary sensor, or measured in a direction contained in the central plane of the primary air gap in the case of a linear sensor],
each stator part being aligned, for a linear sensor, either in a given plane surface parallel to OX or in a given cylindrical surface whose axis is OX and, for a rotary sensor, in a given cylindrical surface whose axis corresponds to the axis of rotation of the movable part, the sensor being provided in addition with at least one movable part equipped with at least one piece of soft magnetic material and at least one magnet joined to that piece, the movable magnetic piece or pieces being displaced parallel to the ferromagnetic stator pieces at a constant minimal distance, the magnet or magnets having their poles parallel to the ferromagnetic pieces of the stator, characterized in that the magnet or magnets of one movable part are prolonged on at least one side by a movable ferromagnetic piece of thickness e, in the direction of magnetization of a magnet, such that 0.1 L less than e less than 0.9 L,
the magnet or magnets being displaced parallel to the ferromagnetic stator pieces at a constant minimal distance and having magnetization perpendicular to OX and a length equal to at least Xc in the direction of displacement OX
[Xc being measured over the mean radius of the magnet in the case of a magnet of semiannular form or the length in the case of a straight magnet].
According to a first modification, the magnet or magnets is or are partly embedded in a cavity of a movable ferromagnetic yoke on the stator side, having a depth e such that 0.1 L less than e less than 0.9 L, where L is the thickness of the magnet in the magnetization direction.
According to a second modification, the magnet or magnets is or are disposed side-by-side with at least one movable ferromagnetic piece of thickness e in the direction of magnetization of a magnet, such that 0.1 L less than e less than 0.9 L, the magnet or magnets having a length equal to at least Xc in the direction of displacement OX.
Advantageously, the movable member has a magnet partly embedded in a cavity situated substantially at the center of the movable ferromagnetic yoke, the cavity and the magnet having a length equal to at least Xc and preferably equal to Xc+F+2 Exe2x80x2 in the direction OX, in the case of a magnet of semiannular form this length is measured over the mean radius of the magnet, F is the length of the air gap in the direction OX in which the probe is placed, Exe2x80x2 ranges between e/4 and E, where E is the distance measured perpendicular to OX between the ferromagnetic stator pieces and the bottom of the cavity.
According to a particular embodiment, the movable member has a ferromagnetic yoke of length equal to at least 3 Xc+F+6 Exe2x80x2 measured along OX.
Advantageously, the ferromagnetic stator pieces have a length Xs substantially equal to at least Xc+2 Exe2x80x2, measured along OX, in the case of a rotary sensor the length is measured over the mean radius of a magnet.
Preferably the depth e of a cavity of the movable ferromagnetic yoke is determined in such a manner as to achieve a sensor of maximum linearity, preferably with 0.3 L less than e less than 0.8 L.
According to one modification, the cavity in a movable ferromagnetic yoke is constructed in such a manner as to obtain the largest possible e/L ratio, preferably with 0.5 L less than e less than 0.9 L, while conserving a small linearity error, preferably of less than 3%.
According to a preferred embodiment, the thickness e, measured in the direction of magnetization of a magnet, of a movable ferromagnetic piece placed side-by-side with a permanent magnet is determined in such a manner as to achieve a sensor of maximum linearity, preferably with 0.3 L less than e less than 0.8 L.
Preferably the thickness e, measured in the direction of magnetization of a magnet, of a movable ferromagnetic piece placed side-by-side with a permanent magnet, is provided in such a manner as to obtain the largest possible e/L ratio, preferably with 0.5 L less than e less than 0.9 L, while conserving a small linearity error, preferably of less than 3%.
According to a particular embodiment, the L/E ratio is greater than 0.5 and preferably greater than or equal to 0.75, where E is the distance measured perpendicular to OX between the ferromagnetic stator pieces and the bottom of the cavity.
Advantageously, the Xs/e ratio is greater than 5 and preferably greater than or equal to 8.
Accordingly to one modification, the ferromagnetic stator pieces, the magnet or magnets and the movable ferromagnetic piece or pieces have the same length Z, which is preferably greater than or equal to 3 E, measured along the axis perpendicular to the direction of magnetization and to the direction of displacement OX.
According to another modification, the fixed and movable parts respectively are partly exchanged for movable and fixed parts respectively.
According to a modified embodiment, the stator structure is composed of four parallelepiped ferromagnetic pieces defining two pairs of secondary air gaps which intersect at a center point. Each secondary air gap being equipped with a magnetoresensitive probe.
According to one modification, the movable part can be displaced in two directions OX and OY, and it is composed of a ferromagnetic yoke in which there is partly embedded a permanent magnet polarized in the direction of the primary air gap that separates the stator parts from the movable yoke.
According to one modification, the movable part can be displaced in both directions OX and OY, and it is composed of a permanent magnet polarized in the direction of the primary air gap that separates the stator parts from the movable yoke, disposed side-by-side with at least one movable ferromagnetic part.